The factors that cause the cracking of welded steel pipes in the flattening test are analyzed from the aspects of experimental equipment, quality of materials and personnel operation, and the main reasons for the cracking of welded pipes in the flattening test are obtained.
The flattening test is one of the common methods for the quality inspection of welding defects of welded steel pipes (Figure 1). The method is to place the defective welded pipe in a parallel plate, and use a press or other methods to uniformly process the welded pipe. Stop the press until the size reaches the target value, and detect the deformation of the welded pipe. Whether the welded pipe meets the standard requirements should be determined according to the test results. Since the cracking of the welded pipe is related to the production site, it is necessary to understand the construction site of the welded pipe.
Figure 1 Welded steel pipes
1. The construction of welded pipes
The production process of welded pipes is simple, and it has the advantages of high production efficiency, low costs, rapid development, and many varieties. They are widely used in construction on site. However, the welding seam is the weakest part of the welded steel pipe due to the process. The strength is lower than that of the seamless steel pipe of the same specification.
In recent years, the welding process of ERW pipes and spiral welded pipes has been significantly improved due to the significant improvement in the construction process of welded pipes. The welding seam direction of ERW and spiral welded pipes determines the performance difference between the two. The ERW welded pipe is obtained by direct welding, while the spiral welded pipe is made by rolling the pipe with a specific helical angle. ERW pipes have better reliability.
In the welding process, the fiber of the welding seam is one of the characteristics of the welding seam, and the important indicator for judging the welding extrusion force is the metal streamline. If the rising angle of the metal streamline is great, it indicates that the welding extrusion force is great, and more metal is extruded and melted. It can be judged that there is a problem with the welding of the metal at the edge of the plate, so cold welding is likely to occur.
2. Factors affecting the experimental results and analysis of causes of cracking
2.1 Factors affecting the experimental results
2.1.1 Influence of experimental equipment and experimental methods
Among the various schemes for flattening experiments on welded pipes, the experimental equipment and experimental methods are not the same, and most of the flattening experiments are performed on both ends of the welded pipe. In the process of applying pressure, a universal testing machine (Figure 2) or a pressure tensile machine is mainly used to analyze the cracking of the two ends of the welded pipe; a small number of experiments are carried out in the middle of the welded pipe. As a result, there are different degrees of deviation in the test results.
Figure 2 Universal testing machines
2.1.2 Influence of quality of experimental materials
Another major cause of the experimental error is the quality of the materials. The quality will affect the physical and chemical performance indicators of the welded pipe, as well as the quality indicators of the welded pipe. Therefore, during the experiment, different parameters will be selected according to the different quality of the materials, which is also one of the factors that lead to the deviation of the experimental results. During the experiment, if different parameters are selected, the results of the experiment will be directly affected. For example, welded pipes should be used scientifically when low-pressure fluids are transported. When transporting low-pressure fluids such as heating steam and water, materials that meet the requirements of the corresponding standards should be selected. According to the requirements of GB/T 3091-2015 standard, the flattening experiment can be carried out with welded pipe of an outer diameter of not less than 60.3mm and a length of 64mm, and corresponding judgments and design are made according to the experimental results. During the experiment, if there is cracking at the welding seam, it will affect the experimental results. Therefore, materials with good quality should be selected for the experiment.
2.1.3 Influence of operation in the experiment
The experimental results have a lot to do with the operation of the personnel. Improper operations will cause an error in the experimental results. The influence of personnel operation on the experimental results is reflected in the following two aspects:
(1) When the flattening experiment is carried out, the deviation of the experimental results will be caused due to the different operating habits and professional abilities of the operators. If the operator's professional ability is good and the operation process is standardized, the accuracy of the obtained experimental results will be high; the selection of welded pipes will also be representative, which can reflect the accurate evaluation of the welding process on the production level of welded pipes.
(2) The degree of operation standard of the operator in the experiment process directly affects the reliability of the experiment result. Because the operation procedure of the experiment is relatively complicated, if the operator's operation cannot be strictly carried out according to the operation procedure, it is easy to cause deviation from the experimental results, and even lead to wrong results.
2.2 Factors causing cracking of welded pipes
2.2.1 Influence of impurities inside the welded pipe
To meet the requirements of the flattening test of the welded pipe, ensure the overall performance of the welded pipe, and prevent the welded pipe from cracks or other defects during the experiment; when the flattening test of the welded pipe is carried out and the cause analysis of cracking is carried out, it is necessary to study substance in the welded pipe’s section layer. For example, analyze sulfide, and clarify the influence of form, content and distribution of sulfide on experimental results. In addition, it is necessary to carry out all-round detection of impurities in the welded pipe, and select high-quality materials that meet the requirements of relevant standards so as to avoid the influence of the internal impurities in the welded pipe’s flattening test on the experimental results.
2.2.2 Influence of materials of welded pipes
There is allotropy in steel, that is, when the temperature and pressure are changed, the crystal structure of steel will change, from one lattice to another lattice. The main allotropes of iron and steel are a iron, B iron and y iron. Among them, a iron cracks are the most seriously, and the cracked section will form longitudinal cracks after cracking, which will seriously reduce the value of the welded pipe and affect the use of the welded pipe. B iron and y iron also crack, but mainly transverse cracks, no serious longitudinal cracks, which have relatively little impact on the use of welded pipes. From the overall crack degree of welded pipes, the value of welded pipes is closely related to the degree of cracks and is inversely proportional, that is to say, the higher the degree of cracks is, the lower the value of the welded pipe becomes. On the contrary, the lower the degree of cracks is, the higher the value of the welded pipe becomes.
